Fix compact rev sql on case of missing

[mhmxs]

There is a very little chance that compactRevSQL is returning an empty result, which breaks other query executions. Coalesce ensures the query always returns a number.

[brandond]

There is some discussion about this at #374. The compact_rev_key row should never be absent. How are you able to reproduce this state? Even if you manually delete that row with sqlite CLI, it will be recreated on startup.

[mhmxs]

Thanks @brandond you are right, I forgot to start the driver and faced with the problem. So i made the fix. Weeks later i added the forgotten start for other reason, and didn't connect the dots it solves this issue. Sorry my bad :)